An interface for DALI control signals which comprises a transmitting channel and a receiving channel both of which can be operated with a common current source is known from DE 10 2009 016 904 B4. The circuit according to the prior art is shown in FIG. 1.
In the known circuit, corresponding optocouplers U2, U1, which each form part of a branch for transmitting or respectively receiving, are provided both for the reception of DALI signals and also for the transmission of DALI signals. Both branches are fed from the common current source Q2, Q3, R3, R4. The circuit further comprises an energy store, which is illustrated in FIG. 1 as the capacitor C2.
The known interface is designed for a communication according to the DALI standard in which a specified DC-voltage is present on the lines in the case of an inactive bus. This specified DC-voltage is reduced respectively only in the case of a signal communication, while the constant DC-voltage is again present when no signals are being communicated.
According to the prior art, the capacitor C2 is charged by the DC-voltage present in the bus. This is meaningful here because, if a signal communication according to the DALI standard is taking place, precisely the voltage present in the bus falls to (logical) zero or respectively to the voltage which is defined for the low-level voltage. This can be detected directly in the return channel (transmitting branch) of the circuit.
“Return channel” relates to the channel away from the interface as the channel for the transmitting mode of the interface. The “transmitting branch” is accordingly the signal path of the interface used for the transmission of signals.
However, if signals according to a protocol in which the voltage is zero (or very low by comparison with the DALI standard) in the non-operating state of the bus are received by the interface instead of DALI signals, the known interface may then not be suitable for this. An example for such a standard is the so-called DSI standard.
The reason for this is that, by contrast with the DALI standard, according to the DSI standard, no voltage or respectively a low voltage is present in the case of an inactive bus (the “low level”, that is, the low value for the transmission of a first logical state, for example, 0, is specified at <6.5 V). The voltage in the bus is raised only in the case of a transmission of a DSI signal.
Consequently, if a DSI signal arrives at the terminal for the operating device, that is, at the secondary side of the known interface, the voltage jumps abruptly from the value for the first logical state, for example, < 6.5 V, to a specified DC-voltage, for example, 10-15 V (high-level, that is, the voltage value which is interpreted as the second logical value, for example, 1). It is now.necessary for the incoming signal to be detected immediately in order to guarantee a reliable detection of the DSI signal. In the case of DSI, the transmission takes place with a Manchester coding, that is, one data bit is transmitted through a change from low-level to high-level (logical 0) or respectively a change from high-level to low-level (logical 1).
However, in this context, the capacitor C2 from the known circuit acts in a disturbing manner, because the falling edge (logical 1) or respectively the first bit of the DSI signal cannot be reliably detected by the known interface.
This is because, after adopting the high-level (for approximately 833 μs), the capacitor C2 is partially charged as a result of the 2 mA input current source. In the case of a decline of the bus voltage to below 6.5 V, the capacitor C2 is further charged. In consequence, current also flows in the optocoupler U2 of the receiving branch, and the first logical state (for example, 1) cannot therefore be detected at the optocoupler output of the optocoupler U2 immediately after falling below 6.5 V. The capacitor C2 is in fact still partially charged even after the undercutting of the low-level and, in the non-charged or partially charged state, bridges the Zener diode Z1, which otherwise immediately interrupts the current flow in the optocoupler U2 when the Zener voltage (low level) is undercut.